1. Field of the Invention
The present invention relates to a device for extruding a grooved cylindrical core particularly used as a central cylindrical strand of an optical cable, with each groove following a helical path around the longitudinal axis of this strand and containing one optical waveguide or optical fiber.
2. Description of the Prior Art
A device for manufacturing a rubber tube having helical peripheral threads or stripes is disclosed in French Pat. No. 1,002,545 wherein a sole extruder includes an extruding screw axially centered behind a central member located in a conical bore of a stationary first die through which extrusion material is forced. A small end of this stationary die has a small cylindrical bore determining the outside diameter of the tube or core to be extruded, including the grooves. The stationary die cylindrical bore is extended axially by a short cylindrical bore of a second die that is rotatable about the central member. The rotating die cylindrical bore carries channels to form cylindrical grooves in the smooth surface of the cylindrical core discharged from the stationary die. The grooves are helically formed by rotation of the second die.
In this prior art device, the grooved core is obtained in two separate stages. The first stage consists of producing a cylindrical core with no grooves in the stationary die. The second stage consists of furrowing grooves in the still malleable material of the cylindrical core via the rotating die, causing several disadvantages. For example, core groove profile is only coarsely gauged since the rotating die acts upon partially cooled material. Also, high material pressures are exerted on the rotating die and particularly on the junction between the stationary die cylindrical bore and the rotating die channelled bore which can cause sealing problems, such as material return and leakage between the two dies. Furthermore, these imperviousness problems are augmented by the very mounting of the rotating die which is lodged in the front of the device by means of a roller bearing behind the rotating die and around a shoulder of the device framework. With time and wear, significant play can creep in between the two dies, which reduces the accuracy of the desired gauging still further.